Measurements of the zeta potential of a particle or molecule reflect the state of charge at the hydrodynamic plane where the particle diffuses in the bulk fluid of a suspension. This diffusion is a random process dependant on particle size; when an electric field is applied there is also a directed component in the field direction and this causes a velocity that is linearly dependant on the zeta potential for low fields which do not cause distortion of the fluid around the particle. Measurement of the so called electrophoretic mobility (EPM) can be used to determine the potential. Although absolute measurements are important and can be used to predict dispersion stability, in many important cases it is changes that arise from some treatment of the sample that are observed. Such a change can occur when, for example, a positively charged macromolecule such as a protein binds to the surface of a negatively charged particle so reducing the effective EPM. Observing this reduction in EPM enables the binding process to be quantified and related to the presence of specific ligands on the particle, or the particular ionic constitution of the suspending fluid, such as its pH value or background salt concentration. Current instruments make measurements of individual samples typically of a few hundred microliters of sample, and can measure automatically sequences of samples if they are provided from an autosampler or autotitrator. A typical instrument representing the current ‘state of the art’ is described in Appendix 1 of this document.
To rapidly assess many different samples, representing perhaps different binding proteins, or different concentrations of these, and different ligand particles, a faster method would be very useful.